Enamels are well known in the prior art and are used, especially, for coating glass substrates, such as glazing in the motor-vehicle industry or the building industry. They make it possible to form decorative layers or inscriptions, conductive layers, protective layers, especially layers for protecting the adhesive layers or adhesive beads intended for mounting glazing in body openings from ultraviolet radiation, masks, especially masks for concealing current-collecting strips of heating grids or for concealing the above mentioned adhesive beads and similar products.
Before application to the substrate and firing, these enamels used for coating glass substrates are generally formed from a powder comprising a glass frit, which is included to provide a glassy matrix, and pigments, which are used as colorants. The pigments also possibly form part of the frit. Both the frit and the pigments are generally based on metal oxides. Also, a medium or “vehicle” is used to allow application and temporary adhesion of the enamel to a substrate. The medium, chosen according to the intended use of the enamel, must ensure that the particles of the frits and pigments used are properly in suspension and must be consumed at the latest during firing of the enamel. This medium may include solvents, diluents, oils, resins and the like.
One problem with existing enamelled glass substrates is the difficulty of recycling such products, particularly in the manufacture of glass substrates in the form of sheets or glazings. Since such products must meet many requirements in terms of coloration, light and/or energy transmission, etc., it is important in fact that the glass waste reintroduced in ground form or cullet into the melting furnaces used for manufacturing new glazings does not disturb the glass compositions normally already present and does not give products that do not meet the necessary requirements. Although non-enamelled glass cullet can thus be reintroduced into a furnace for manufacturing float glass in amounts generally of about 20-30% by weight of the charge of the furnace, enamelled glass cullet, on the other hand, generally causes the appearance of undesirable residual colorations or of residual unmelted islands in the glass sheets formed. Consequently, the amount of enamelled glass cullet that can be reintroduced into these furnaces does not generally exceed 2 to 3% by weight of the charge of the furnaces. Usually, the enamelled glass cullet constitutes from 0.1 to 0.5% by weight of the charge.
An improved enamel composition and an enamelling process are already known from WO 98/25864, which makes it possible to obtain improved enamelled glass substrates, particularly enamelled substrates which can be recycled (especially in the manufacture of glazing) more easily than the existing enamelled substrates. This composition comprises as pigments one or more manganese compounds and has a firing temperature, also called melting point, of less than 750° C. In motor-vehicle applications, the composition is preferably chosen so as to obtain a black enamel having the following colorimetric coordinates, in absolute values:                L*≦5; |a*|≦2 and |b*|≦2 (and, preferably,        |a*|≦1 and |b*|≦1).        
This is generally the case in the compositions described in the above document, which essentially comprise Bi2O3 as intermediate oxide. They are obtained either by adding the manganese compounds in the form of powder to a glass frit powder including Bi2O3, or by a process, called the “direct process,” by melting a mixture of suitable raw materials (including the manganese compounds) in order to obtain the desired combination of frit and pigments.
On the other hand, the black coloration sought after in motor-vehicle applications is not obtained systematically with other types of recyclable compositions. In particular, tests carried, out by replacing all or some of the Bi2O3 with a less expensive oxide, such as zinc oxide, have not made it possible to obtain the desired black coloration, whether the Bi2O3 is replaced with a less expensive oxide in the glass frit powder before adding the manganese compounds in the first process described in the previous paragraph, or whether the raw material or materials used for obtaining Bi2O3 is or are replaced with one or more raw materials used for obtaining a less expensive oxide in the mixture of raw materials in the direct process described in the preceding paragraph.
Thus, there is a need to develop recyclable black enamel compositions which are less expensive than previous bismuth-based compositions. This need is satisfied by the present invention.